Earth is a dynamic planet in which convection takes place on the scale of thousands of kilometers. Because Earth is mostly solid (except for its liquid-iron outer core), this convection causes deformation of solid rocks by plastic flow. At the core–mantle boundary (CMB), 2900 km deep, seismologists have discovered that seismic waves travel faster in certain directions. This seismic anisotropy appears to be related to the deformation of the constituent minerals. To understand the deformation mechanisms of mineral phases at this depth, researchers from Yale and UC Berkeley re-created the ultrahigh pressures of the deep Earth at ALS Beamline 12.2.2 while conducting in situ x-ray diffraction experiments to probe changes in crystal orientations.

Rocks Flow in the Deep Earth

Earth is remarkably plastic. Our planet is slowly cooling as heat is transferred from its interior to the surface via convection. Convection not only manifests itself in the plastic deformation of mineral phases due to stress, but it also drives plate tectonics.

Of particular interest is the D'' zone, a layer roughly 200 km thick that lies just above the CMB. This layer exhibits a seismic discontinuity: shear waves (S-waves) increase in velocity a few hundred km above the CMB. In addition, the layer exhibits large topographic variations, lateral variability, and directionally-dependent properties (anisotropy). Numerical modeling and laboratory experiments have suggested that the seismic anisotropy may result from the deformation-induced alignment of the constituent mineral crystal lattices. Miyagi et al. (2010) demonstrated that at the extreme conditions of the lower mantle, such alignment does in fact occur and would produce anisotropy consistent with seismic observations.

Cutaway diagram of the Earth showing interior layers (not to scale). The Earth is solid except for the liquid-iron outer core.

The Earth's mantle is a roughly 2800-km-thick, viscous rocky layer that lies between the liquid-iron outer core and the relatively thin, solid crust. Elementally, the mantle consists mostly of oxygen, silicon, and magnesium, often in the form of magnesium silicate (MgSiO3). In the lower mantle, this magnesium silicate has a crystal structure known as "perovskite." However, at the lowermost mantle, the MgSiO3 is compressed into a new phase known as "post-perovskite" (pPv). Because this phase is unstable below 127 GPa, it has proven difficult to study in the laboratory.

The diamond-anvil cell (DAC) has long been used to study materials at high pressures. Only recently, however, has the DAC been applied (by the UC Berkeley group) to produce pressure and to impose stress while the incoming x-rays are brought in orthogonal (radial) to the compression direction. This particular apparatus is called the radial DAC, or rDAC. When stress is applied to a polycrystal (a solid made up of many randomly oriented crystals), individual crystals deform preferentially along slip planes. This results in crystal rotations that lead to crystallographic preferred orientation ("texture") in the polycrystal, which can be derived from analysis of x-ray diffraction images.

Left: The radial diamond anvil cell (rDAC) is used to apply pressure and deviatoric stress to deform material. Right: Diffraction image observed at 150 GPa. From such images, mineral phases, elastic strains, and crystallographic preferred orientation of crystallites can be quantified.

To understand deformation of MgSiO3 pPv, rDAC experiments were performed at the high-pressure Beamline 12.2.2 of the ALS. To synthesize the pPv phase, MgSiO3 glass was compressed to 148 GPa and was laser heated at ~3500 K for ~10 minutes. The pressure was then increased in four steps to 185 GPa. At each step, in situ x-ray diffraction images were collected to document the evolution of pressure, differential stress, and texture. Inverse pole figures (IPFs) show the probability of finding the pole (i.e., the normal) to a crystallographic lattice plane in the compression direction. In contrast to previous experiments, the group observed that at large strains, a preferred orientation pattern evolves with a maximum at the [001] pole. Comparison of the IPFs with the results of polycrystal plasticity simulations shows that the observed 001 texture is compatible with slipping along {001} lattice planes and 40% compressive strain.

Inverse pole figures (IPFs) illustrate the evolution of preferred orientation of crystals during deformation at 148, 164, and 185 GPa. The corners of the diagram correspond to the [001], [010], and [100] pole directions. Scale bar is in multiples of random distribution (m.r.d.), where m.r.d. = 1 is random orientation (dark blue) and a higher m.r.d. number indicates stronger texture. Far right: The IPF for a polycrystal plasticity simulation, for slipping along the (001) plane and 40% compressive strain, shows excellent agreement with the data.

When this dominant deformation mechanism is applied to large-scale geodynamic convection in the deep Earth, it predicts elastic properties in the CMB region, with fast S-waves polarized parallel to the boundary, which is just what seismologists observe. These results open new possibilities for modeling anisotropy evolution at extreme conditions, linking microscopic mineral physics properties to large macroscopic observations.

Currently, the research team has been working to develop resistive heating techniques in the rDAC experiments capable of reaching temperatures of up to 2000 K, which can be further expanded upon by simultaneous laser heating. This technological advancement is not only relevant for academic investigations in mineral physics but also adds valuable tools for studying materials at extreme conditions.

Research funding: National Science Foundation, Bateman Fellowship (Yale University), and the Carnegie/DOE Alliance Center (Carnegie Institution of Washington). Operation of the ALS is supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences.